Cancer is caused by genetic changes (mutations) in cells that lead to uncontrolled cell growth and tumor formation. These mutations can be inherited or, more commonly, acquired during a person's lifetime due to various factors. Causes of cancer include interactions between genetic factors and external agents such as:

• Physical carcinogens like ultraviolet and ionizing radiation.
• Chemical carcinogens such as tobacco smoke, asbestos, alcohol, aflatoxin, and arsenic.
• Biological carcinogens like infections from certain viruses (HPV, hepatitis B and C, Epstein-Barr virus), bacteria (Helicobacter pylori), or parasites.
• Lifestyle factors including tobacco use, alcohol consumption, unhealthy diet, physical inactivity, and exposure to air pollution.
• Chronic inflammation and certain hormonal and metabolic imbalances also play roles.

Gene mutations can instruct cells to grow uncontrollably, fail to stop growing, or make mistakes in repairing DNA errors, all of which contribute to cancer development. The accumulation of these mutations over time, combined with age-related decline in cellular repair, increases cancer risk. Not all cancer causes are known, and multiple risk factors often interact in complex ways to cause cancer.